The present invention relates to a continuous process of extruding and mechanically dispersing a polymeric resin in an aqueous or non-aqueous medium.
Stable aqueous dispersions of polyepoxide-amine resins that are ionically charged are known. Such resins are described, for example, by McCollum et al. in U.S. Pat. No. 5,114,552, incorporated herein by reference. McCollum et al. discloses that an organic polyepoxide can be rendered cationic by reaction with a primary or secondary amine in the presence of acid to form cationically charged acid salt groups, or by reaction with tertiary amines to form quaternary ammonium salts. These resins are also crosslinkable by virtue of the incorporation of pendant hydroxyl groups or primary or secondary amine groups, which are reactive with a capped polyisocyanate crosslinker. The crosslinked resin is dispersed in an acidified aqueous medium, and the solids content is adjusted with the further addition of water. The extent of cationic salt formation is preferably selected so that a stable dispersion of the cationic polymer is formed, although an external cationic surfactant can also be added. Such stable aqueous dispersions are useful as curable electrocoating compositions.
The preparation described by McCollum et al. is time-consuming because it is carried out in a batch reactor. Furthermore, the reaction requires considerable amounts of organic solvent, which have to be removed.
The preparation of thermoplastic epoxy resins by continuous processes are known. For example, U.S. Pat. No. 4,612,156, incorporated herein by reference, describes the preparation of a high molecular weight phenoxy resin by mixing a diepoxy compound (e.g., a diglycidyl ether of bisphenol A) with a difunctional compound that reacts with the epoxy groups (e.g., bisphenol A) in the presence of a catalyst, and feeding the mixture into a twin screw reactive extruder by way of a single screw extruder or a pump, thus forming an epoxy resin having a weight average molecular weight of about 30,000 Daltons. Such a resin can be ground and sieved, then dispersed in water in the presence of a surfactant to form an aqueous dispersion (sometimes referred to as a powder slurry) which is useful in coating applications.
It would therefore be an advance in the art to be able to prepare a stable aqueous dispersion of a resinous material in a continuous process directly from an extruder without first having to solidify, then grind, then sieve the resin. It would be a further advance in the art to form curable electrocoating dispersions by a continuous method that requires little or no ancillary solvent.
The present invention addresses the problems in the art by providing a continuous method for preparing a stable dispersion or emulsion comprising the step of merging into a mechanical disperser a stream of a molten or liquid disperse phase containing a polymer with a stream of a molten or liquid continuous phase to form a dispersion or an emulsion, wherein
a) the continuous phase is substantially immiscible with the disperse phase; and
b) the polymer is self-dispersable or either the disperse phase or the continuous phase or both contains a stabilizing amount of a surfactant; and
c) the polymer is continuously extruded in an extruder that is coupled to the mechanical disperser.
In another aspect, the present invention is a method of preparing a high internal phase ratio emulsion without phase inversion comprising the steps of:
a) continuously merging into a disperser a continuous phase liquid stream having a flow rate of R1 and a disperse phase liquid stream having a flow rate of R2; and
b) mixing the merged streams in the substantial absence of a surfactant and at a mixing rate sufficiently constant to form the high internal phase ratio emulsion without phase inversion;
wherein the disperse phase liquid stream contains a self-dispersing polymer or prepolymer and wherein R2:R1 is such that the mean volume average particle size of the high internal phase ratio emulsion is less than 2 xcexcm or the polydispersity is less than 2.
The process of the present invention provides a means of preparing a stable aqueous dispersion or emulsion by a continuous process wherein an extruder is coupled to a continuous mechanical dispersion process.